The present invention relates to an electrochemical device linking a plurality of bonded bodies in which a pair of electrodes are oppositely arranged with an electrolyte membrane in between, and particularly to an electrochemical device suitable for a fuel cell, a fuel sensor and the like.
Currently, various primary batteries and secondary batteries are used as an electric source of electronic devices. As one of indicators exhibiting characteristics of these batteries, there is an energy density. The energy density is an energy cumulative amount per unit mass of a battery.
As miniaturization and high performance of the electronic devices have been developed in recent years, a high capacity and a high output of the electric source, in particular, the high capacity of the electric source is increasingly necessitated. Thus, it has been hard to supply a sufficient energy to drive the electronic devices with the use of the conventional primary batteries and the conventional secondary batteries. Therefore, it is urgently needed to develop a battery having a higher energy density. Fuel cells attract attention as one of candidates having a higher energy density.
The fuel cell has a structure in which an electrolyte is arranged between an anode (fuel electrode) and a cathode (oxygen electrode). A fuel is supplied to the fuel electrode, and air or oxygen is supplied to the oxygen electrode. This results in redox reaction in which the fuel is oxidized by oxygen in the fuel electrode and the oxygen electrode, and part of chemical energy of the fuel is converted to electric energy and extracted.
Various types of fuel cells have been already proposed and experimentally produced, and part thereof is practically used. These fuel cells are categorized into an Alkaline Fuel Cell (AFC), a Phosphoric Acid Fuel Cell (PAFC), a Molten Carbonate Fuel Cell (MCFC), a Solid Electrolyte Fuel Cell (SOFC), a Polymer Electrolyte Fuel Cell (PEFC) and the like according to the electrolyte used.
FIG. 12 illustrates a structure of a conventional PEFC. The PEFC has a bonded body (MEA; Membrane Electrolyte Assembly) 110 in which a fuel electrode 112 and an oxygen electrode 113 are arranged with an electrolyte membrane 111 composed of a solid polymer electrolyte in between. A unit in which the bonded body 110 is sandwiched between separators (not illustrated) configures one unit cell.
A voltage capable of being extracted from the one unit cell is about 0.3 V to 0.8 V, and this voltage is not enough to be used singly. Therefore, in general, a fuel cell stack in which a plurality of unit cells are stacked is used. Meanwhile, for the use of mobile devices, a thin structure is preferred, and thus it is often the case that a plane stacked structure in which a plurality of unit cells are two-dimensionally arranged in line or in a plurality of lines, and such a plurality of unit cells are electrically connected in series is adopted.
In the two-dimensionally arranged bonded bodies 110, the electron transfer distance between adjacent two bonded bodies 110 is larger than that in vertically stacked bonded bodies. Thus, the arrangement and the current collection structure of the bonded bodies 110 are important to decrease resistance of all cells. That is, as illustrated in FIG. 13(A), in the case where the bonded bodies 110 are vertically stacked with a separator 120 in between, average transfer distance L is small and electron transfer cross-sectional area S is large, and thus electric resistance generated in the separator 120 can be kept small, resulting in an advantageous structure for flowing a large current. Meanwhile, as illustrated in FIG. 13(B), in the case where the bonded bodies 110 are two-dimensionally arranged by linking the bonded bodies 110 by a connection plate 130, by contraries, the average transfer distance L is large and the electron transfer cross-sectional area S is small, resulting in a disadvantageous structure for extracting a large current (for example, refer to Non Patent Document 1).
Conventionally, for example, the following structure has been proposed. In the structure, electricity generated in a unit cell is collected by using a Z-shaped connection plate, and adjacent unit cells are electrically connected in series, and thereby the electron transfer distance is shortened (for example, refer to Patent Document 1).    Non Patent Document 1: “Function chemistry of electron and ion Vol. 4: All about Polymer Electrolyte Fuel Cell,” Edited by Hiroyuki Uchida and three authors, NTS Inc., 2003, pp. 143-145)    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2002-56855